1. Field of the Invention
The present invention relates to an image forming apparatus, such as a color copying machine, a printer, and a facsimile machine, of a tandem method.
2. Discussion of the Background
A known image forming apparatus, for example a color copying machine of a tandem method, which has an image forming mechanism including a conveying belt which is rotated in a sub-scanning direction and plural image carriers which the conveying belt is brought into contact by a predetermined contact force.
The image forming mechanism includes, for each image carrier, a charging device which uniformly charges the surface of the image carrier, an exposing device which forms an electrostatic latent image by exposing a predetermined image, a developing device which forms a predetermined toner image by making a developer containing a toner of each color adhere to the electrostatic latent image, and a transferring device which transfers the toner image onto a sheet conveyed on a conveying surface of the conveying belt by applying a transferring bias between the image carrier and the transferring device.
In such an image forming apparatus, it has been known that image densities of toner images of a predetermined pattern, which are directly transferred from the plural image carriers onto the conveying belt at a predetermined timing, are respectively detected, and on the basis of the detection results, image forming conditions, such as developing biases by the developing devices or transferring biases by the transferring devices etc., are respectively set for the plural image carriers.
With respect to detection of the image densities of such toner images for setting the image forming conditions, there is known an image forming apparatus in which one density sensor is installed at the downstream side of the image carrier positioned at the most downstream side in a sheet conveying direction, and the densities of the toner images which are transferred from respective image carriers onto the conveying belt are detected by the density sensor. In the image forming apparatus, detection of the image densities of plural toner images on the conveying belt transferred from the plural image carriers by an inexpensive structure has been enabled by realizing detection of the image densities of the toner images with one sensor.
Further, there is known an image forming apparatus in which toner images on image carriers are transferred onto a sheet which is conveyed on the conveying surface of a conveying belt by way of an intermediate transfer belt. In such an image forming apparatus, the image densities of the toner images of a predetermined pattern are detected at predetermined timings, and image forming conditions such as developing biases by developing devices or transferring biases by transferring devices are set on the basis of the detection results.
Further, there has been known an image forming apparatus in which plural density sensors are installed for respective image carriers, and the image densities of toner images which have been transferred from respective image carriers onto a conveying belt are detected immediately after having been transferred. In this image forming apparatus, highly reliable detection values can be obtained by detecting the image densities of the toner images immediately after the toner images have been transferred from the respective image carriers.
In the image forming apparatus in which the density sensor is installed at the downstream side of the image carrier positioned at the most downstream side in the sheet conveying direction, detection of the image densities of toner images on the conveying belt (an intermediate transfer member) has been realized by an inexpensive structure. However, the toner images on the conveying belt other than the one which has been transferred from the image carrier positioned at the most downstream side in the sheet conveying direction, pass the contacting portions between the image carriers and the conveying belt (the intermediate transfer belt), which are at downstream side of the respective transferring portions, and therefore what is called a reverse transfer of toner occurs, such that the toner which has been transferred onto the conveying belt (the intermediate transfer belt) once is transferred onto the image carrier again. Thus, there is a possibility that the image densities detected by the image sensor are the ones of the toner images in which the reverse transfer has occurred, reducing reliability in the detected image densities of the toner images.
When the image forming operation is performed under image conditions set according to the image densities of reduced reliability, the reproducibility of the formed image is decreased.
In the image forming apparatus in which the plural density sensors are installed for respective image carriers, the highly reliable image densities of the toner images can be obtained; however, the cost of the apparatus is increased because the density sensors are installed for respective image carriers.
In addition, Japanese Laid-Open Patent Publication No. 11-102091 discloses an image forming apparatus in which density sensors are installed at the upstream side and the downstream side in the moving direction of the conveying surface of a conveying belt for respective image carriers in order to consider the amount of a reverse transfer of a toner image, and image forming conditions are set according to the image densities of toner images which have been detected by those density sensors before and after transferring. However, according to the technique disclosed in the above publication, although it is possible to set the image forming conditions considering the reverse (repeated) transfer, the cost of an apparatus is increased because the plural density sensors are installed for respective image carriers.
Accordingly, preferred embodiments of the present invention provide an image forming apparatus that forms an image of high reproducibility, without being influenced by a so called reverse transfer of toner, by an inexpensive structure.
According to a preferred embodiment of the present invention, an image forming apparatus of a tandem method, includes, a conveying member configured to provide to a recording member a conveying force in a sub-scanning direction, a plurality of image carriers configured to contact the conveying member with an equal interval between contacting portions of the plurality of image carriers and the conveying member, a contact force adjusting mechanism configured to freely adjust a contact force of the conveying member to the plurality of image carriers between a predetermined contact force and a decreased contact force, a plurality of toner image forming devices configured to form toner images of a predetermined pattern on the plurality of image carriers respectively, a length of the pattern in the sub-scanning direction being set shorter than a length of the equal interval between the contacting portions of the conveying member and the plurality of image carriers, a plurality of transferring devices configured to transfer the toner images on the plurality of image carriers onto the conveying member, respectively, and a contact force changing device configured to change the predetermined contact force by the contact force adjusting mechanism to the decreased contact force before respective tips of the toner images on the conveying member pass subsequent contacting portions of the contacting portions between the plurality of image carriers and the conveying member, so that respective parts of the toner images on the conveying member are not transferred back onto subsequent image carriers of the plurality of image carriers.
Further, the image forming apparatus further includes, an image density detecting device configured to detect image densities of the toner images on the conveying member, and an image forming condition setting device configured to set an image forming condition on a basis of the image densities detected by the image density detecting device.
Further, the image density detecting device detects the image densities of the toner images on the conveying member before and after the predetermined contact force by the contact force adjusting mechanism is changed to the decreased contact force, and the image forming condition setting device sets the image forming condition on a basis of the image densities detected by the image density detecting device before and after the predetermined contact force by the contact force adjusting mechanism is changed to the decreased contact force.
Further, the plurality of transferring devices transfer the toner images by applying transferring biases between the conveying member and the plurality of image carriers respectively, and the image forming condition setting device sets respective electric potential strengths of the transferring biases.
Further, the plurality of toner image forming devices include the plurality of developing devices holding developer including toner, and form respectively the toner images by making the developer adhere to the plurality of image carriers by applying developing biases between the plurality of developing devices and the plurality of image carriers respectively, and the image forming condition setting device sets respective electric potential strengths of the developing biases between the plurality of developing devices and the plurality of image carriers.
Further, the plurality of toner image forming devices include plurality of toner containers containing toner and plurality of developing devices holding developer including the toner supplied from the plurality of toner containers, and form the toner images by making the developer adhere to the plurality of image carriers by applying developing biases between the plurality of developing devices and the plurality of image carriers respectively, and the image forming condition setting device sets respective toner amounts supplied from the plurality of toner containers to the plurality of developing devices.
According to another preferred embodiment of the present invention, an image forming apparatus of a tandem method, includes, an intermediate transfer member configured to rotate in a sub-scanning direction and to intermediately carry toner images to be transferred onto the recording member, arranged to oppose a conveyed recording member, a plurality of image carriers configured to contact the intermediate transfer member with an equal interval between contacting portions of the plurality of image carrier and the intermediate transfer member, a contact force adjusting mechanism configured to freely adjust a contact force of the intermediate transfer member to the plurality of image carriers between a predetermined contact force and a decreased contact force, a plurality of toner image forming devices respectively configured to form toner images of a predetermined pattern on the plurality of image carriers respectively, a length of the pattern in the sub-scanning direction being set shorter than a length of the equal interval between the contacting portions of the intermediate transfer member and the plurality of image carriers, a plurality of transferring devices configured to transfer the toner images on the plurality of image carrier onto the intermediate transfer member respectively, and a contact force changing device configured to change the predetermined contact force by the contact force adjusting mechanism to the decreased contact force before respective tips of the toner images on the intermediate transfer member respectively pass subsequent contacting portions of the contacting portions between the plurality of image carriers and the intermediate transfer member, so that respective parts of the toner images on the intermediate transfer member are not transferred back onto corresponding subsequent image carriers of the plurality of image carriers.
According to a preferred embodiment of the present invention, a method of forming an image with an image forming apparatus of a tandem method including a conveying member to provide to a recording member a conveying force in a sub-scanning direction, and a plurality of image carriers configured to contact the conveying member at a predetermined contact force with an equal interval between contacting portions of the plurality of image carrier and the conveying member, the method includes, forming toner images of a predetermined pattern on the plurality of image carriers respectively, a length of the pattern in the sub-scanning direction being set shorter than a length of the equal interval between the contacting portions of the conveying member and the plurality of image carriers, transferring the toner images onto the conveying member, and decreasing the predetermined contact force of the conveying member to the plurality of image carriers before respective tips of the toner images on the conveying member pass subsequent contacting portions of the contacting portions between the plurality of image carriers and the conveying member, so that respective parts of the toner images on the conveying member are not transferred back onto subsequent image carriers of the plurality of image carriers.
Further, the method of forming, further includes, detecting image densities of the toner images on the conveying member, and setting an image forming condition on a basis of the detected image densities.
Further, the detecting of image densities includes detecting the image densities of the toner images on the conveying member before and after decreasing the predetermined contact force, and the setting of image forming condition sets the image forming condition on a basis of the image densities detected before and after decreasing the predetermined contact force.
Further, the transferring of toner images transfers the toner images by applying transferring biases between the conveying member and the plurality of image carriers, and the setting of image forming condition sets respective electric potential strengths of the transferring biases.
Further, the forming of toner images includes making developer adhere to the plurality of image carriers by applying developing biases between plurality of developing devices and the plurality of image carriers, and the setting of image forming condition sets respective electric potential strengths of the developing biases between the plurality of developing devices and the plurality of image carriers.
Further, the forming of toner images includes making developer adhere to the plurality of image carriers by applying developing biases between plurality of developing devices and the plurality of image carriers, and the setting of image forming condition sets respective toner amounts supplied from plurality of toner containers to the plurality of developing devices.
According to a preferred embodiment of the present invention, a method of forming an image with an image forming apparatus of a tandem method including an intermediate transfer member to intermediately carry toner images to be transferred onto the recording member, arranged to oppose a conveyed recording member and to rotate in a sub-scanning direction, and a plurality of image carriers to contact the intermediate transfer member with an equal interval between contacting portions of the plurality of image carrier and the intermediate transfer member, the method includes, forming toner images of a predetermined pattern on the plurality of image carriers respectively, a length of the pattern in the sub-scanning direction being set shorter than a length of the equal interval between the contacting portions of the intermediate transfer member and the plurality of image carriers, transferring the toner images onto the intermediate transfer member, and decreasing the predetermined contact force of the intermediate transfer member to the plurality of image carriers before respective tips of the toner images on the intermediate transfer member pass subsequent contacting portions of the contacting portions between the plurality of image carriers and the intermediate transfer member, so that respective parts of the toner images on the intermediate transfer member are not transferred back onto subsequent image carriers of the plurality of image carriers.